1. Field of the Invention
The present invention relates to information reproducing and information recording/reproducing methods for a magneto-optical recording medium such as a magneto-optical disk, a magneto-optical tape and a magneto-optical card, in particular, a magnetically induced super resolution medium capable of magnetically induced super resolution reproduction, for use in a magneto-optical recording/reproducing device, and also relates to a magneto-optical reproducing device.
2. Description of Related Art
A magneto-optical disk has recently become popular as an external recording medium for a computer. In the magneto-optical disk, a record bit with a submicron size is formed on the medium through application of an external magnetic field and irradiation of a laser beam. Thus, the recording capacity can be remarkably increased as compared with that of a conventionally used external recording medium such as a flexible disk and a hard disk.
Meanwhile, information is recorded in a rewritable magneto-optical disk by heating a magneto-optical recording film thereon through beam irradiation, so as to change the magnetization direction (record bits) in the heated portion in accordance with an external magnetic field corresponding to the information to be recorded. In a reproducing operation, the magneto-optical recording film is irradiated with a light beam, so as to reproduce the recorded information by making use of the Kerr effect in that the plane of polarization of reflected light is rotated in accordance with the magnetization direction.
Such a magneto-optical disk has been regarded as a main memory for storing a mass of data in the rapidly developing multimedia, and there are increasing demands for further increasing its capacity. In order to increase the recording capacity of a magneto-optical disk, namely, in order to increase the recording density, it is necessary to further minimize the record bits as well as further decrease a pitch between the bits.
However, a conventional general magneto-optical recording/reproducing operation is limited by the size of a light beam (beam spot) on a medium. The beam spot can be made smaller in order to reproduce data from a bit with a smaller size than the diameter of the beam spot. Still, the minimization of the beam spot is limited because the beam spot is limited by the wavelength of a light source and the numeral aperture of an object lens.
In order to achieve high density recording, a magnetically induced super resolution medium (MSR medium) and a recording/reproducing method utilizing the MSR medium, in which data in a record bit smaller than a beam spot can be reproduced, are proposed (Japanese Patent Application Laid-Open Nos. 1-143041 (1989), 3-93058 (1991), 4-271039 (1992) and 5-12731 (1933)). In the recording/reproducing method, a recording medium including lamination of a plurality of magnetic layers respectively having different magnetic characteristics depending upon the temperature is used, so that the effect attained by a small beam spot can be attained by utilizing the temperature distribution caused in the beam spot on the recording medium. In this manner, recorded data can be definitely read even when the record bits are smaller than the beam spot.
Such an MSR medium is classified into the following three types: a front aperture detection (FAD) type as is shown in FIG. 1, in which a high-temperature area in a laser beam spot works as a masking portion and a low-temperature area works as an aperture from which bits are read (Japanese Patent Application Laid-Open No. 1-143041 (1989)); a rear aperture detection (RAD) type as is shown in FIG. 2, in which a low-temperature area in a laser beam spot works as a masking portion and a high-temperature area works as an aperture from which bits are read (Japanese Patent Application Laid-Open No. 3-93058 (1991)); and an RAD double-mask type as is shown in FIG. 3, in which a high-temperature area and a low-temperature area in a laser beam spot work as masking portions and an intermediate-temperature area works as an aperture from which bits are read (Japanese Patent Application Laid-Open No. 4-271039 (1992)).
Information recorded in the MSR medium of these types is conventionally reproduced by a technique disclosed in Japanese Patent Application Laid-Open No. 4-258831 (1992).
In this reproducing technique, a reference signal is previously recorded at a predetermined position such as an inner-most area and/or an outer-most area of the MSR medium. In a reproducing operation, the reference signal is first read, so as to determine an optimal reproducing power on the basis of the reference signal.
In this conventional technique, the determined optimal reproducing power is set and retained at a previously fixed value. However, the MSR medium may have a warp, and hence, the shape of a beam spot can be changed because of the so-called tilt, namely, because an incident beam, which is required to vertically enter the recording surface, actually enters the recording surface at an angle. As a result of this change, the reproducing power is decreased. Furthermore, a local blur caused by a fingerprint or the like on the surface of the MSR medium and variation in the characteristics of the recording surface due to fluctuation in the film thickness, the components and the like can also decrease the reproducing power. Such factors to decrease the reproducing power vary depending upon the location on the recording medium. In addition, the temperature on the medium is changed in a time lag between the correction of the reproducing power and an actual reproducing operation. Therefore, it is disadvantageously difficult to set optimal reproducing conditions.
Moreover, the optimal reproducing power means a reproducing power for attaining the maximum signal amplitude in the magnetically induced super resolution reproduction of a record bit in an aperture formed in a beam spot as is shown in FIGS. 1 through 3.
The MSR medium includes, however, a portion where information is magneto-optically recorded, that is, the so-called MO portion, such as a data portion, as well as a portion where information is recorded by forming an emboss bit on the medium, that is, the so-called emboss bit portion, such as an SM portion and an ID portion.
The information recorded in the emboss bit portion can be read by using a much smaller power than the optimal reproducing power.
In other words, when the optimal reproducing power is fixed at a predetermined value, the medium is heated by an unnecessarily large reproducing power in a tracking servo operation and a reproducing operation of the emboss bit portion such as the SM portion and the ID portion. As a result, the MSR medium can be degraded and a semiconductor laser used for irradiation can be degraded, which is one of the factors to shorten their lifetime.